It is known that transversally excited gas lasers, containing carbon dioxide at atmospheric pressure, make it possible to generate laser beams of high energy. However, the use of such gas lasers is impossible in many cases because the energy is not uniformly radiated. The uniformity of radiation depends on the homogeneity of the electrical discharge. It is known to improve the homogeneity of the discharge by using a pre-ionization of the gaseous laser medium. Among various possibilities of pre-ionization, a photo-pre-ionization has been found highly effective.
A direct photo-ionization of carbon dioxide, nitrogen or helium molecules contained in the gas laser is conditional upon the photons having energies exceeding 14 eV. Such photons, at atmopsheric pressure, have a penetration depth limited to 30 microns, so that a space pre-ionizatin is not possible. In order to overcome this difficulty, supplementary substances with a low ionization potential can be used. This method has the great advantage that penetration depth of the photons is independent of the gas pressure and that, accordingly, a genuine space pre-ionization is possible at high gas pressures, too.
The choice of such a supplementary substance is, however, greatly limited by the following requirements:
The ionisation potential of the supplementary substance must be smaller than about 7 eV. PA1 The supplementary substance must have a high vapor pressure at room temperature. PA1 The supplementary substance and its possible decomposition products must not react with the laser gas and must not cause a reduction of the excited level. PA1 The supplementary substance and its possible decomposition products must be transparent to the laser light at its wavelength and must not disturb the electrical discharge. PA1 Furthermore, a light source of high power is needed which has powerful emission near the ionization energy of the supplementary substance.
Various supplementary substances have already been proposed. However, their ionization potentials are too high to permit a single-stage hoto-ionization.